No.  "You can't get there from here."

Whoever did that would have had to totally scrap the 2203/2204 preamp and design one from scratch. 

The JCM800 had a unique (at least at the time) preamp. 

For decades before the JCM800 the only different between the HI and LO inputs on an amp was that the LO input was attenuated to weaken the incoming signal. 

With the JCM, the HI SENS input actually runs the signal through a single gain stage and then straight to the LO SENS input, which then runs the signal through a more traditional preamp design.  For all practical purposes, the HI SENS input is nothing but a boost going into the LO SENS input.   Everything is serial/linear, the signal runs through the amp in a straight line  HI input-->Boost-->LO input-->Preamp-->Poweramp.  To make the HI and LO two separate channels with their own Volume controls would really mean running parallel circuits, at least for a little while (for them to have individual Master Volumes, it would men running separate preamps all the way to the PI)

If you want two unique channels, that's where the 2205/2210 series comes in. 

Here is the 2203 schematic.  Note the HI SENS jack runs to V1A, from there the signal goes out the plate, through the .022 cap and runs straight to the physical, mechanical LO SENS jack.  And that jack is switched, plug a cable in LO and it will disconnect the HI circuit from it. 

Ok I didnt know that it was a huge deal, I was misinformed, Im quite happy with this amp

Kevin